Interpretive Summary: Field populations of plant viruses contain many different variants of the same virus species. These variants coexist in the same space and time, suggesting that individual virus isolates may have evolved mechanisms to physically separate themselves from one another. Three potential isolating mechanisms were investigated using three genetically defined strains of wheat streak mosaic virus (WSMV). An assay was developed which could determine the presence of different WSMV strains in mixed infections. Mixed infections were easily established by simultaneous inoculation of wheat plants with strain mixtures. In contrast, when closely related WSMV strains were inoculated seven days apart, the first virus strain to be inoculated excluded the second virus strain. Thus, the phenomenon of cross-protection operates among strains of WSMV. Furthermore, the distribution of WSMV strains within coinfected plants was non-uniform, indicating that viral populations within a plant are subdivided. Strain mixtures were commonly resolved into single infections upon transmission by individual wheat curl mites, the natural vector of WSMV. This result demonstrates that vector transmission may reduce the genetic diversity within a single virus isolate and that mixed infections are likely transitory in the field. Collectively, these three mechanisms provide sufficient genetic isolation for individual viral lineages to evolve independently of one another to produce the complex array of variants observed under field conditions.

Technical Abstract:
Cross-protection and vector transmission bottlenecks have been proposed as mechanisms facilitating genetic isolation of sympatric viral lineages. Molecular markers were used to monitor establishment and resolution of mixed infections with genetically defined strains of wheat streak mosaic virus (WSMV). Two closely related WSMV strains from the U. S. (Type and Sidney 81) exhibited reciprocal cross-protection in wheat, confirming this classic phenomenon as a mechanism of genetic isolation. In contrast, cross-protection between either U. S. strain and the divergent El Batan 3 strain from Mexico was only partially effective and unilateral. Distribution of WSMV strains in individual leaves of plants supporting a mixed infection of Type and Sidney 81 was spatially non-uniform. Strain distribution in individual tillers of coinfected plants was also heterogeneous, with some containing either Type or Sidney 81 alone and some econtaining both. Transmission by wheat curl mites, acquiring virus from source plants simultaneously infected with both Type and Sidney 81, often resulted in test plants bearing only a single WSMV strain. Spatial subdivision of virus strains within coinfected plants likely contributed to vector transmission bottlenecks during acquisition. Collectively, these three distinct mechanisms enhance genetic isolation of individual viral lineages, and together with stochastic forces, may explain generation and maintenance of genetic diversity in field populations.